1. Field of the Invention
The present invention relates to packet transmission employing automatic repeat request (ARQ) under environment of mobile communication.
2. Description of the Related Art
Recently, as the Internet has widely spread, a demand on data communication under mobile communication environment has increased remarkably, and, achievement of mobile communication having a higher data rate and a larger capacity than those of a next generation mobile communication system (IMT-2000) is expected. In data communication field, as data transmission at various data rates and quality of service is required, a packet transmission system in which data is divided into packets of fixed information amounts which are then transmitted is effective.
Further, in a data communication field, error free transmission must be guaranteed, differently from voice communication or the like. Accordingly, automatic repeat request (ARQ; see “Automatic repeat request error control schemes,” S. Lin, D. J. Costello, and M. J. Miller, IEEE Trans. Commun. Mag., vol. 22, pp. 5–17, December 1984) is essential. In ARQ, a packet encoded with an error detection code added to a information signal sequence is transmitted from a transmission end, and, at a reception end, a re-transmission request for the packet is made toward the transmission end when error has been found in the received packet by using the error detection code. At the transmission end, the packet for which the re-transmission request was made is transmitted again, and this operation is repeated until no error is found in the reception end. Thereby, error free transmission can be achieved.
However, according to ARQ, such a re-transmission request occurs frequently when a channel condition is not satisfactory, and, thereby, a transmission performance is remarkably degraded. In order to solve this problem, Hybrid ARQ in which error probability is reduced as a result of employing a forward error correction code in addition to the error detection code is proposed. Hybrid ARQ includes Type-I in which the re-transmission packet is identical to the initial transmission packet, and Type-II/III in which the re-transmission packet is not identical to the initial transmission packet. In Type-I, study has been made such that when a received packet includes error at a reception end, the packet is stored, and, after a re-transmission packet is received, both the packets are combined together symbol-by-symbol. Thereby, packet combining such that a signal-to-noise ratio (SNR) can be improved may be achieved (see “A Diversity Combining DS/CDMA system with convolutional encoding and Viterbi decoding,” S. Souissi and S. Wicker, IEEE Trans. Veh. Techol., vol. 44, No. 2, pp. 304–312, May 1995).
On the other hand, in Type-II/III, original code is restored by combining the already transmitted packet and re-transmission packet which are punctured by the different puncturing pattern. Thereby, it is possible to improve the coding rate (see “Rate-compatible punctured convolutional codes and their applications,” J. Hagenauer, IEEE Trans. Commun. vol. 36, pp. 389–400, April 1988).
Further, according to IMT-2000, information transmission of maximum 2 Mbps is rendered. However, a higher transmission rate is needed in consideration of future demand. Therefore, employing of a variable rate system such that the transmission rate can be improved without widening the frequency band by changing the modulation level and coding rate appropriately when a situation of a transmission channel is satisfactory is being studied (see “Performance of Symbol Rate and Modulation Level Controlled Adaptive Modulation System,”, Toyoki Ue, Seiichi Sampei, and Norihiko Morinaga, Technical Report of IEICE, SST95-21, CS95-28, RCS95-30, May 1995). Further, with regard to ARQ employing a variable rate system, a study has been made (see “Type-II Hybrid ARQ Scheme using Punctured Convolutional Code with Adaptive Modulation Systems,”, Masashi Naijoh, Seiichi Sampei, Norihiko Morinaga, and Yukiyoshi Kamino, Technical Report of IEICE, CS96-27, RCS96-20, May 1996). According to this study, at a reception end, a situation of a transmission channel is estimated, and, based on a result of the estimation, transmission parameters such as the modulation levels, coding rate and symbol rate are determined.
On the other hand, in consideration of commonality with IMT-2000, CDMA is a promising candidate as an access method, and, a composite technique of transmission power control and site diversity which are inherent to CDMA, with ARQ is demanded. According to CDMA, a same frequency is used in common with other users. Accordingly, when a power larger than a required one is transmitted, the number of users who can use the same frequency band is reduced. Therefore, in order to prevent the transmission power from exceeding a required one, transmission power control is performed. For example, a transmission power control signal (for example, a TPC command for increasing or decreasing the power at a transmission end) is transmitted from a reception end to the transmission end such that the power to be transmitted from the transmission end is controlled so that power received by the reception end may be kept constant.
Further, in CDMA system, as one frequency is repeatedly used, site diversity is performed such that signals from a plurality of base stations are received/transmitted in a time overlapped manner.